Recently, Bluetooth, wireless personal area network (WPAN) technology has been developed, which allows audio or video data to be exchanged between devices by configuring a wireless network between a relatively small number of digital devices in limited places such as homes or small-scaled companies. WPAN can be used to exchange information between a relatively small number of digital devices in a relatively close distance, and enables communication of low power and low cost between digital devices. IEEE 802.15.3 (Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs) approved on Jun. 12, 2003 defines specification of MAC and PHY layers of high-rate WPAN.
FIG. 1 is a diagram illustrating a configuration example of WPAN. As shown in FIG. 1, the WPAN is a network configured between devices within a limited place such as home. The network is configured by direct communication between devices to enable seamless exchange of information between applications.
Referring to FIG. 1, the WPAN is comprised of at least two user devices 11 to 15, wherein one of the user devices is operated as a coordinator 11. The coordinator 11 serves to provide basic timing of the WPAN and control quality of service (QoS) requirements. Examples of the user devices include computer, PDA, notebook computer, digital TV, camcorder, digital camera, printer, mike, speaker, headset, barcode reader, display, and cellular phones. All digital devices can be used as the user devices.
The WPAN is an ad hoc network (hereinafter, referred to as ‘piconet’) which is not previously designed but formed if necessary without assistance of a central infra. The procedure of forming one piconet will be described as follows. The piconet starts in such a manner that a device that can be operated as a coordinator performs a function of a coordinator. All devices start a new piconet or perform scanning before association with the existing piconet. Scanning means that a device collects and stores information of channels and searches the presence of the existing piconet. A device which has been instructed from an upper layer to start a piconet forms a new piconet without association with a piconet which is previously formed on a channel. The device starts a piconet by selecting a channel having little interference based on the data acquired during scanning and broadcasting a beacon through the selected channel. In this case, the beacon is timing allocation information, information of other devices within the piconet, and control information broadcasted by a coordinator to control and manage the piconet.
FIG. 2 illustrates an example of a superframe used in the piconet. Timing control in the piconet is performed based on the superframe. Referring to FIG. 2, each superframe is started by a beacon transmitted from a coordinator. A contention access period (CAP) is used to allow devices to perform contention based transmission of commands or asynchronous data. A channel time allocation period may include a management channel time block (MCTB) and a channel time block (CTB). The MCTB is a period which transmits control information between a coordinator and a device or between devices, and the CTB is a period which transmits asynchronous or isochronous data between a device and a coordinator or between other devices. In each superframe, the number, the length and the position of the CAPs, the MCTBs, and the CTBs are determined by the coordinator and transmitted to other devices within the piconet through the beacon.
If a random device within the piconet needs to transmit data to the coordinator or another device, the device requests the coordinator to provide a channel time for data transmission, and the coordinator allocates the channel time to the device within the range of available channel resources. If a contention access period exists within the superframe and the coordinator allows data transmission in the contention access period, the device can transmit a small amount of data through the contention access period without the channel time allocated from the coordinator.
If the number of devices within the piconet is small, since channel resources which each device transmits data are sufficient, no special problem occurs in channel time allocation. However, if channel resources are insufficient due to a large number of devices, or if a specific device continues to occupy channels to transmit data of large capacity, such as moving pictures, even though the number of devices is small, a problem may occur in that communication is impossible as channel resources are not allocated to other devices having data to be transmitted. Also, even though channel resources are allocated to other devices, a problem may occur in that the channel resources are smaller than capacity of data stored in the other devices.